Thermal dye sublimation transfer also called thermal dye diffusion transfer is a recording method in which a dye-donor element provided with a dye layer containing sublimable dyes having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, heated with a thermal printing head provided with a plurality of juxtaposed heat-generating resistors, whereby dye from the selectively heated regions of the dye-donor element is transferred to the receiver sheet and forms a pattern thereon, the shape and density of which is in accordance with the pattern and intensity of heat applied to the dye-donor element.
A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which is covered with a dye layer, which contains the printing dyes. Usually an adhesive or subbing layer is provided between the support and the dye layer. Normally the opposite side is covered with a slipping layer that provides a lubricated surface against which the thermal printing head can pass without suffering abrasion. An adhesive layer may be provided between the support and the slipping layer.
The dye layer can be a monochrome dye layer or it may comprise sequential repeating areas of different colored dyes like e.g. of cyan, magenta, yellow and optionally black hue. When a dye-donor element containing three or more primary color dyes is used, a multicolor image can be obtained by sequentially performing the dye transfer process steps for each color.
It is always desirable to transfer as much dye as possible with the lowest thermal energy in said thermal dye sublimation transfer systems. The amount of dye which can be transferred from a dye-donor element to a receiving element by thermal dye transfer depends upon the dye transfer efficiency. It is known to add so-called thermal solvents to the dye-donor element in order to increase the dye transfer efficiency and thus to obtain enhanced dye transfer densities. Thermal solvents are non-hydrolyzable organic compounds that are solid at ambient temperature but molten at elevated temperatures. They have a melting point between 40.degree. C. and 300.degree. C., preferably between 40.degree. C. and 150.degree. C. In molten state they act as a solvent within the element in which they are contained. These compounds are known under such different names like thermal solvents, melt-formers, melt-modifiers, eutectic formers, plasticizers, softeners, and thermal development and diffusion-promoting agents.
Various classes of thermal solvents have been described for use in thermal dye transfer donor elements, for example, in EP 318944, EP 318945, EP 390044, JP 56/89985, JP 59/222391, JP 60/44392, JP 60/56590, JP 61/286199, JP 62/108086, JP 62/283176, JP 02/3384, JP 02/25387, JP 02/151485 and JP 03/10891.
Diphenyl compounds with various linking groups between the two phenyl nuclei have also been described as thermal solvents in dye-donor elements. Examples of linking groups described are esters (see EP 318945 and JP 61/286199), ketones (see EP 318944), (sulfon)amides (see EP 318944) and ethers (see JP 02/3384 and JP 02/25387).
In EP 318945 non-substituted diphenyl carbonates are used as thermal solvent in the dye-donor element. Although these compounds have a beneficial effect on dye transfer they adversely affect the stability of the donor element. When dye-donor elements having such dye layers containing non-substituted diphenyl carbonates as thermal solvents have been rolled up and stored for any length of time such that the backcoat of one portion of the donor element is held against the dyecoat of another portion, sticking of the backcoat to the dyecoat occurs and migration of the dye takes place leading to a loss of density of any prints eventually made using that donor element. Further said thermal solvents cause crystallization of the dye.